1. Field of the Invention
The present invention relates to a phase-shifting mask, and more particularly to a phase-shifting mask which has a phase shifter formed by thermal deformation of an organic photoresist followed by CMP (Chemical Mechanical Polishing) to prevent pattern errors at an 180.degree./0.degree. phase boundary.
2. Description of the Prior Art
In general, the photolithography process used in manufacturing semiconductors uses a mask which consists of a patterned opaque layer provided on a transparent substrate. Phase shift masks have recently been introduced in order to improve optical resolving power and correct degraded resolution due to optical interference at the edges of the opaque layer. In particular, a phase shifter is provided at the edge of the opaque layer to shift the phase of incident light to achieve these improvements. Generally, a phase difference of 180.degree. between the phase shifter and the transparent substrate is preferred and is obtained when the thickness of the phase shifter satisfies the following formula: ##EQU1##
where, n is a refractive index of the phase shifter, .lambda. is a wavelength of light from a light source, and n.sub.0 is an ambient refractive index. Typically n.sub.o is the refractive index of air, which is unity.
FIG. 1 illustrates a plan view of a conventional phase-shifting mask, FIG. 2 illustrates a sectional view of the phase-shifting mask in FIG. 1 along section line 4--4', and FIG. 3 illustrates a sectional view of the phase-shifting mask in FIG. 1 length-wise along section line 5--5'.
As shown in FIGS. 1-3, the conventional alternating phase-shifting mask includes a plurality of light shielding layers 2 and phase shifters 3 formed on the transparent substrate 1 between the light-shielding layers 2. It should be noted that phase shifters 3 are not formed between each adjacent pair of light shielding layers 2, but between alternate pairs of light shield layers 2. Further, the light shielding layers 2 are not provided along the entire periphery of the phase shifter 3, but only adjacent portions of light shielding layers 2.
That is, as shown in FIG. 2, in some cases, the peripheries of the phase shifters 3 are part of the light shielding layers 2. In this case, the edges of the phase shifters 3 do not directly contact transparent substrate 1. Therefore, as shown in FIG. 2, the phase-shifting mask does not affect the intensity of light transmitted through the mask.
However, the desired circuit pattern frequently requires the peripheries of the phase shifters 3 to contact transparent substrate 1 directly (see FIG. 3). On the other hand, in order to obtain an optimal phase-shifting effect, thicknesses of the phase shifters 3 should be uniform and satisfy equation (1) above. Accordingly, at the peripheries of phase shifters 3, where no light shielding layers 2 have been formed and phase shifter 3 directly contacts substrate 1, the amplitude of incident light is abruptly changed from positive to negative, or vice versa, and the phase is also inverted.
Therefore, as shown in FIG. 4, the light intensity at a semiconductor substrate (reference number "9" in FIG. 5) drops to near zero at the boundary of the phase shifter 3. As a result, the edge portion behaves like an opaque region so that, after exposure, regions 8 of positive photoresist remain on semiconductor substrate 9 as residual patterns.
To cope with aforementioned problems in the conventional phase-shifting mask, different techniques have been developed.
FIGS. 6a-6c show sectional views for manufacturing a conventional phase-shifting mask incorporating one such technique in which the edges of the particular phase shift mask have sloped sides which prevent abrupt phase shifting. A method for forming this particular conventional phase-shifting mask will now be explained.
As shown in FIG. 6a, light shielding layers 2 and first phase shifters 3 are formed on a transparent substrate 1 using the same process explained in association with FIGS. 1 and 3. A second phase shifter material 4 is then formed on the entire substrate surface by thermal oxidation.
As shown in transverse and lengthwise views of FIGS. 6b and 6c, respectively, the second phase shifter material 4 is dry etched to form second phase shifters 6 as sidewall spacers located on the sides of first phase shifters 3. As a result, abrupt amplitude changes at the edges of the phase shifter can be prevented. Consequently, residual photoresist patterns shown in FIG. 4 are not formed.
However, light shielding layers 2 are susceptible to deformation during high temperature process steps. Further, in case the first and second phase shifters are formed of the same material, it is difficult to identify the etch end point. Therefore, it can be difficult to precisely control the thickness of the phase shifter.
An alternative example for forming a conventional phase-shift mask is shown in FIGS. 7a and 7b. In this case, light shielding layers 2 and first phase shifters 3 are first formed according to the same method explained above with reference to FIGS. 1-3. Next, as shown in FIG. 7a, phase shifters 3 are etched by successive photo masking steps to form stepped portions of varying sizes.
As shown in FIG. 7b, the structure shown in FIG. 7a is subjected to a heat treatment to reflow the stepped portion to form phase shifter 3 with sloped side surfaces.
However, the above desired conventional phase-shifting masks have the following problems. First, the light shielding layers are susceptible to deformation during the heat treatment step. Moreover, etch end point detection is difficult when the first and second shifters are formed of the same material. As a result, the mask substrate can be damaged.
Second, phase shifters made of a dielectric material can be charged up during electron beam direct writing of the phase shifter material.
Third, a uniform phase-shifting effect of the phase shifter is difficult to obtain because light shielding layers underlie the phase shifters.
Fourth, the process used to fabricate the conventional phase-shifting mask is complicated.